It is known that an internal combustion using gasoline as fuel reduces the amount of nitrogen oxide (NOx) in an exhaust gas when hydrogen gas is supplied in addition to gasoline. A technology disclosed, for instance, by Japanese Patent Laid-open No. 2004-116398 determines the hydrogen addition ratio so as to reduce the NOX exhaust amount, and operates an internal combustion engine by injecting gasoline and hydrogen in accordance with the determined ratio.
[Patent Document 1]
Japanese Patent Laid-open No. 2004-116398
[Patent Document 2]
Japanese Patent Laid-open No. Hei 6-200805
However, if the internal combustion engine includes a canister for adsorbing vaporized fuel (fuel vapor) in a gasoline tank, the amount of vaporized fuel purged from the canister to an intake path cannot be accurately controlled. Therefore, even when hydrogen gas is supplied, unstable combustion results.
It is difficult to accurately control the air-fuel ratio in an engine cylinder particularly when a large amount of vaporized fuel is purged from the canister to the intake path. Consequently, the air-fuel ratio is likely to shift toward the lean side or rich side. Unstable combustion then results, thereby causing emission and driveability to deteriorate.